Isomerization process

ABSTRACT

An improved process is provided for catalytic isomerization of monocyclic methyl-substituted aromatic hydrocarbon compounds of from 8 to 10 carbon atoms contained in a feedstock also containing ethylbenzene in a reaction zone maintained under conditions such that said isomerization is accomplished in the vapor phase. The improvement in the process comprises a first step wherein the temperature is maintained at a level which provides from about 50 mole percent to about 75 mole percent conversion of said ethylbenzene for a period of time of from about 24 hours to about 96 hours, and a second step wherein the temperature is maintained at a level which provides from about 15 mole percent to less than about 50 mole percent conversion of said ethylbenzene.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a new and useful improvement in aprocess for the isomerization of monocyclic methyl-substituted aromatichydrocarbon compounds of from 8 to 10 carbon atoms contained in afeedstock also containing ethylbenzene. The process comprises the use ofa specific catalyst in a vapor phase reaction, said catalyst containinga crystalline aluminosilicate zeolite characterized by a constraintindex, hereinafter defined, within the approximate range of 1 to 12. 2.Description of the Prior Art

The catalytic arrangement of alkyl groups present in alkyl aromatichydrocarbons to provide one or more products suitable for use in thepetroleum and chemical industries has heretofore been effected by a widevariety of catalysts. Acidic halides such as aluminum chloride, aluminumbromide, boron trifluoride-- hydrogen fluoride mixtures, etc. have beenused in the rearrangement of alkyl benzenes to provide valuableintermediates which find utility in the synthesis of rubber, plastic,fibers and dyes. Other catalysts which have been used include solidsiliceous cracking-type catalysts such as silica-alumina and clays andplatinum deposited on silica-alumina. Although various catalysts possessone or more desired characteristics, a majority of catalysts heretoforeemployed suffer from several disadvantages. Acidic halides such asaluminum chloride, for example, are partially soluble in the feedmaterial and are easily lost from the catalyst zone. Catalysts of thistype are also uneconomical because of their extreme corrosiveness andrequirement for recovery from the effluent products. Other catalysts ofthe heterogeneous type, such as silica-alumina, platinum on alumina,etc., do not possess sufficient acidity to provide effective conversionand necessitate the use of relatively high temperatures above the orderof 427° C. to 510° C. through the entire isomerization process.Prolonged high temperatures of this order for these catalyst materialsfrequently lead to excessive coke formation which lowers the yield ofdesired product and necessitates frequent regeneration of the catalystto remove coke. This results in reducing on-stream time and leads tohigh catalyst consumption due to loss of catalyst activity.Heterogeneous catalyst such as the crystalline aluminosilicates, bothnatural and synthetic, possess sufficient acidity but suffer thedisadvantage of poor selectivity and aging as evidenced by "coke" makeand the excessive amounts of disproportionated product formed inisomerization reactions.

A process in the art for isomerization of xylene is Octafining,extensively discussed in the literature as exemplified by:

1. Pitts, P. M., Connor, J. E., Leun, L. N., Ind. Eng. Chem., 47, 770(1955).

2. Fowle, M. J., Bent, R. D., Milner, B. E., presented at the FourthWorld Petroleum Congress, Rome, Italy, June 1955.

3. Ciapetta, F. G., U.S. Pat. No. 2,550,531.

4. Ciapetta, F. G., and Buck, W. H., U.S. Pat. No. 2,589,189.

5. Octafining Process, Process Issue, Petroleum Refinery, 1st Vol. 38(1959), No. 11, Nov., p. 178.

The catalyst for use in such process is platinum on silicaalumina.

An improved catalyst for use in Octafining plants is taught by U.S. Pat.No. 3,856,872 to be of the ZSM-5 type of zeolite, whereby the processoperates at high space velocities. Further, a process utilizing ZSM-5type zeolites in acid form for vapor-phase conversion of a feedstockcontaining mixed C₈ aromatics in the absence of added nitrogen is taughtby U.S. Pat. No. 3,856,873. Even in such processes, especially when thecatalyst has increased acid activity, there is a loss of xylene, theprimary isomerization product, presumably due in part todisproportionation of xylenes and/or transalkylation of xylenes withethylbenzene which may be present in the reaction system.

It is hereby proposed and demonstrated that by conducting a first stepof the process at a temperature which provides from about 50 molepercent to about 75 mole percent conversion of ethylbenzene present inthe reaction system for a period of time of from about 24 hours to about96 hours, a second step of the process may be conducted at a temperaturewhich provides only from about 15 mole percent to less than about 50mole percent, preferably from about 15 mole percent to about 30 molepercent, conversion of said ethylbenzene with reduced occurrence ofundesirable by-product reaction and with reduced xylene loss.

SUMMARY OF THE INVENTION

This invention relates to a new and useful improvement in vapor phaseisomerization of monocyclic methyl-substituted aromatic hydrocarbons offrom 8 to 10 carbon atoms contained in a feedstock which also containsethylbenzene. The isomerization reaction is carried out in the presenceof a catalyst composition containing a crystalline aluminosilicatezeolite characterized by a constraint index of from about 1 to about 12.The zeolite of the catalyst composition may or may not contain, asreplacement for at least a part of the original alkali metal cations,cations of metal of Group VIII of the Periodic Table of Elements, e.g.,nickel, platinum, iron and/or cobalt. Further, the zeolite of thecatalyst composition may contain, as replacement for at least a part ofthe original alkali metal cations, hydrogen or hydrogen precursorcations. The improvement involves a first process step wherein thetemperature is maintained at a level which provides from about 50 molepercent to about 75 mole percent conversion of ethylbenzene in thefeedstock for a period of time of from about 24 hours to about 96 hours,thereby selectively coking the catalyst to a controlled extent, and asecond process step wherein the temperature is maintained at a levelwhich provides from about 15 mole percent to less than about 50 molepercent conversion of said ethylbenzene.

DESCRIPTION OF SPECIFIC EMBODIMENTS

The catalyst composition useful in this invention contains a crystallinealuminosilicate zeolite characterized by a constraint index of from 1 toabout 12, non-limiting examples of which include ZSM-5, ZSM-11, ZSM-12,ZSM-35 and ZSM-38.

Zeolite ZSM-5 is taught by U.S. Pat. No. 3,702,886, issued Nov. 14,1972, the disclosure of which is incorporated herein by reference. In apreferred synthesized form, the zeolite ZSM-5 for use in the catalystcomposition useful in this invention has a formula, in terms of moleratios of oxides in anhydrous state, as follows:

    (0.9±0.2)M.sub.2/N O:Al.sub.2 O.sub.3 :xSiO.sub. 2

wherein M is selected from the group consisting of a mixture of alkalimetal cations, especially sodium, and tetraalkylammonium cations, thealkyl groups of which preferably contain 2 to 5 carbon atoms, and x isat least 5. Particularly preferred is a zeolite having the formula inthe anhydrous state as follows:

    (0.9±0.2)M.sub.2/n O:Al.sub.2 O.sub.3 :ZSiO.sub.2

wherein Z is from greater than 30 to about 350 or higher.

Zeolite ZSM-11 is taught by U.S. Pat. No. 3,709,979, issued Jan. 9,1973, the disclosure of which is incorporated herein by reference. Inthe as synthesized form, the zeolite ZSM-11 for use in the catalystcomposition useful in this invention has a formula, in terms of moleratios of oxides in the anhydrous state, as follows:

    (0.9±0.3)M.sub.2/n O:Al.sub.2 O.sub.3 :20 to 90 SiO.sub.2

wherein M is a mixture of at least one of the quaternary cations of aGroup V-A element of the Periodic Table and alkali metal cations,especially sodium. The original cations can be present so that theamount of quaternary metal cations is between 10 and 90 percent of thetotal amount of the original cations. Thus, the zeolite can be expressedby the following formula in terms of mole ratios of oxides:

    (0.9±0.2)( xXR.sub.4 +(1- x)M.sub.2/n O):Al.sub.2 O.sub.3 :20 to 90 SiO.sub.2

wherein R is an alkyl or aryl group having between 1 and 7 carbon atoms,M is an alkali metal cation, X is a Group V-A element, especially ametal, and x is between 0.1 and 0.9.

Zeolite ZSM-12 is taught by U.S. Pat. No. 3,832,449, issued Aug. 27,1974, the disclosure of which is incorporated herein by reference. Inthe as synthesized form, the zeolite ZSM-12 for use in the catalystcomposition useful in this invention has a formula, in terms of moleratios of oxides in the anhydrous state, as follows:

    (0.9±0.3)M.sub.2/n O:Al.sub.2 O.sub.3 :20 to 200 SiO.sub.2

wherein M is at least one cation having the valence n, and has acharacteristic X-ray diffraction pattern.

ZSM-35 is described by U.S. Pat. No. 4,016,245, the disclosure of whichis incorporated herein by reference. This zeolite can be identified, interms of mole ratios of oxides and in the anhydrous state, as follows:

    (0.3 to 2.5)R.sub. 2 O:(0 to 0.8)M.sub.2 O:Al.sub.2 O.sub.3 :xSiO.sub. 2

wherein R is an organic cation, M is an alkali metal cation and x isgreater than 8, and is characterized by a specified X-ray powderdiffraction pattern.

In a preferred synthesized form, zeolite ZSM-35 has a formula, in termsof mole ratios of oxides and in the anhydrous state, as follows:

    (0.4 to 2.5)R.sub.2 O:(0 to 0.6)M.sub.2 O:Al.sub.2 O.sub.3 :ySiO.sub.2

wherein R is an organic nitrogen-containing cation derived fromethylenediamine, pyrrolidine, butanediamine or an N-methylpyridiniumcompound, such as for example, the hydroxide, halide, sulfate ornitrate, M is an alkali metal, especially sodium, and y is from greaterthan 8 to about 50.

ZSM-38 is described by U.S. Pat. No. 4,046,859, the disclosure of whichis incorporated herein by reference. This zeolite can be identified, interms of mole ratios of oxides and in the anhydrous state, as follows:

    (0.3 to 2.5)R.sub.2 O:(0 to 0.8)M.sub.2 O:Al.sub.2 O.sub.3 :xSiO.sub. 2

wherein R is an organic cation, M is an alkali metal cation and x isgreater than 8, and is characterized by a specified X-ray powderdiffraction pattern.

In a preferred synthesized form, zeolite ZSM-38 has a formula, in termsof mole ratios of oxides and in the anhydrous state, as follows:

    (0.4 to 2.5)R.sub.2 O:(0 to 0.6)M.sub.2 O:Al.sub.2 O.sub.3 :ySiO.sub.2

wherein R is an organic nitrogen-containing cation derived from a2-(hydroxyalkyl) trialkylammonium compound, wherein alkyl is methyl,ethyl or a combination thereof, M is an alkali metal, especially sodium,and y is from greater than 8 to about 50.

Another zeolite characterized as above and, therefore, useful as acatalyst component for the present process is described in U.S.Application Ser. No. 878,558, file Feb. 17, 1978. This zeolite,possessing a definite distinguishing crystalline structure whose X-raydiffraction pattern shows substantially the significant lines set forthin U.S. Pat. No. 3,702,886 for zeolite ZSM-5, can be identified, interms of mole ratios of oxides in the anhydrous state, as follows:

    (R.sub.2 O, M'.sub.2/n O).sub.W :(Al.sub.2 O.sub.3).sub.X :(SiO.sub.2).sub.Y :(M".sub.2/n O).sub.Z

wherein W/X is from greater than 0.5 to less than 3, Y/X is greater than20 and Z/X is from greater than zero to less than about 100, R is anitrogen-containing cation and n is the valence of M' or M". Thefunction R may include primary amines containing 2 to 10 carbon atomsand ammonium cations, preferably the tetraalkylammonium cation in whichthe alkyl contains from 2 to 5 carbon atoms. The function M' is a metalfrom Group IA of the Periodic Table, ammonium, hydrogen or mixturesthereof. The function M" is a metal, preferably selected from the groupconsisting of rare earth metals (i.e. metals having atomic numbers from57 to 71), chromium, vanadium, molybdenum, indium, boron, mercury,tellurium, silver and one of the platinum group metals, which lattergroup includes platinum, palladium and ruthenium.

Although the zeolites herein described have unusually low aluminacontents, i.e. high silica to alumina ratios, they are very active evenwhen the silica to alumina ratio exceeds 30. The activity is surprisingsince catalytic activity is generally attributed to framework aluminumatoms and cations associated with these aluminum atoms. These catalystsretain their crystallinity for long periods in spite of the presence ofsteam at high temperature which induces irreversible collapse of theframework of other zeolites, e.g. of the X and A type. Furthermore,carbonaceous deposits, when formed, may be removed by burning to restoreactivity. In many environments the zeolites of this class exhibit longtimes on stream between burning regenerations.

An important characteristic of the crystal structure of the zeolites foruse herein is that they provide constrained access to, and egress from,the intracrystalline free space by virtue of having a pore dimensiongreater than about 5 Angstroms and pore windows of about a size such aswould be provided by 10-membered rings of oxygen atoms. It is to beunderstood, of course, that these rings are those formed by the regulardisposition of the tetrahedra making up the anionic framework of thecrystalline aluminosilicate, the oxygen atoms themselves being bonded tothe silicon or aluminum atoms at the centers of the tetrahedra. Briefly,the preferred type catalysts useful in this invention possess, incombination: a silica to alumina ratio of at least about 12; and astructure providing constrained access to the crystalline free space.

The silica to alumina ratio referred to may be determined byconventional analysis. This ratio if meant to represent, as closely aspossible, the ratio in the rigid anionic framework of the zeolitecrystal and to exclude aluminum in the binder or in cationic or otherform within the channels. Although catalysts with a silica to aluminaratio of at least 12 are useful, it is preferred to use catalysts havinghigher ratios of at least about 30.

The present invention provides a highly effective vaporphase-isomerization process with a catalyst, the crystallinealuminosilicate zeolite portion of which, as suggested above, has asmaller pore size than those crystalline aluminosilicates previouslyused for such purpose.

The type zeolites useful in this invention freely sorb normal hexane andhave a pore dimension greater than about 5 Angstroms. In addition, thestructure must provide constrained access to larger molecules. It issometimes possible to judge from a known crystal structure whether suchconstrained access exists. For example, if the only pore windows in acrystal are formed by 8-membered rings of oxygen atoms, then access tomolecules of larger cross-section than normal hexane is excluded and thezeolite is not of the desired type. Windows of 10-membered rings arepreferred, although, in some instances, excessive puckering or poreblockage may render these catalysts ineffective. Twelve-membered ringsdo not generally appear to offer sufficient constraint to produce theadvantageous conversions. Also, structures can be conceived which may beoperative due to pore blockage or other cause.

Rather than attempt to judge from crystal structure whether or not acatalyst possesses the necessary constrained access, a simpledetermination of the "contraint index" may be made by passingcontinuously a mixture of an equal weight of normal hexane and3-methylpentane over a small sample, approximately 1 gram or less, ofcatalyst at atmospheric pressure according to the following procedure. Asample of the catalyst, in the form of pellets or extrudate, is crushedto a particle size about that of coarse sand and mounted in a glasstube. Prior to testing, the catalyst is treated with a stream of air at538° C. for at least 15 minutes. The catalyst is then flushed withhelium and the temperature adjusted between 288° C. and 510° C. to givean overall conversion between 10% and 60%. The mixture of hydrocarbonsis passed at 1 liquid hourly space velocity (i.e., 1 volume of liquidhydrocarbon per volume of catalyst per hour) over the catalyst with ahelium dilution to give a helium to total hydrocarbon mole ratio of 4:1.After 20 minutes on stream, a sample of the effluent is taken andanalyzed, most conveniently by gas chromatography, to determine thefraction remaining unchanged for each of the two hydrocarbons.

The "constraint index" is calculated as follows: ##EQU1##

The constraint index approximates the ratio of the cracking rateconstants for the two hydrocarbons. Catalysts suitable for the presentinvention are those having a constraint index in the approximate rangeof 1 to 12. Constraint Index (CI) values for some typical catalysts,including those useful herein, are:

    ______________________________________                                        Crystalline                                                                   Aluminosilicate   CI                                                          ______________________________________                                               ZSM-5      8.3                                                                ZSM-11     8.7                                                                ZSM-12     2                                                                  ZSM-35     2                                                                  ZSM-38     2                                                                  Beta       0.6                                                                ZSM-4      0.5                                                                H-Zeolon   0.5                                                                REY        0.4                                                                Erionite   38                                                          ______________________________________                                    

It is to be realized that the above constraint index values typicallycharacterize the specified zeolites but that such are the cumulativeresult of several variables used in determination and calculationthereof. Thus, for a given zeolite depending on the temperature employedwithin the aforenoted range of 288° C. to 510° C., with accompanyingconversion between 10% and 60%, the constraint index may vary within theindicated approximate range of 1 to 12. Likewise, other variables suchas the crystal size of the zeolite, the presence of possibly occludedcontaminants and binders intimately combined with the zeolite may affectthe constraint index. It will accordingly be understood by those skilledin the art that the constraint index, as utilized herein, whileaffording a highly useful means for characterizing the zeolites ofinterest, is approximate, taking into consideration the manner of itsdetermination, with the probability, in some instances, of compoundingvariable extremes. However, in all instances, at a temperature withinthe above-specified range of 288° C. to 510° C. the constraint indexwill have a value for any given zeolite of interest herein within theapproximate range of 1 to 12.

The specific zeolites described, when prepared in the presence oforganic cations, are catalytically inactive, possibly because theintracrystalline free space is occupied by organic cations from theforming solution. They may be activated by heating, for example, in aninert atmosphere at 538° C. for one hour, followed by base exchange withammonium salts and by calcination at 538° C. in air. The presence oforganic cations in the forming solution may not be absolutely essentialto the formation of this type zeolite; however, the presence of thesecations does appear to favor the formation of this special type ofzeolite. More generally, it is desirable to activate this type catalystby base exchange with ammonium salts followed by calcination in air atabout 538° C. for from about 15 minutes to about 24 hours.

Natural zeolites may sometimes be converted to this type zeolitecatalyst by various activation procedures and other treatments such asbase exchange, steaming, alumina extraction and calcination, incombinations. Natural minerals which may be so treated includeferrierite, brewsterite, stilbite, dachiardite, epistilbite, heulanditeand clinoptilolite. The preferred crystalline aluminosilicates areZSM-5, ZSM-11, ZSM-12, ZSM-35 and ZSM-38, with ZSM-5 particularlypreferred.

In a preferred aspect of this invention, the catalysts hereof areselected as those having a crystal framework density, in the dryhydrogen form, of not substantially below about 1.6 grams per cubiccentimeter. It has been found that zeolites which satisfy all three ofthese criteria are most desired for the present process. Therefore, thepreferred catalysts of this invention are those having a constraintindex as defined above of about 1 to about 12, a silica to alumina ratioof at least about 12 and a dried crystal density of not less than about1.6 grams per cubic centimeter. The dry density for known structures maybe calculated from the number of silicon plus aluminum atoms per 1000cubic Angstroms, as given, e.g. on page 19 of the article on ZeoliteStructure of W. M. Meir. This paper, the entire contents of which areincorporated herein by reference, is included in "Proceedings of theConference on Molecular Sieves, London, April 1967", published by theSociety of Chemical Industry, London, 1968. When the crystal structureis known, the crystal framework density may be determined by classicalpyknometer techniques. For example, it may be determined by immersingthe dry hydrogen form of the zeolite in an organic solvent which is notsorbed by the crystal. It is possible that the unusual sustainedactivity and stability of this class of zeolite is associated with itshigh crystal anionic framework density of not less than about 1.6 gramsper cubic centimeter. This high density of course must be associatedwith a relatively small amount of free space within the crystal, whichmight be expected to result in more stable structures. This free space,however, is important as the locus of catalytic activity.

Crystal framework densities of some typical zeolites are:

    ______________________________________                                                     Void          Framework                                          Zeolite      Volume        Density                                            ______________________________________                                        Ferrierite   0.28 cc/cc    1.76 g/cc                                          Mordenite    .28           1.7                                                ZSM-5, -11   .29           1.79                                               Dachiardite  .32           1.72                                               L            .32           1.61                                               Clinoptilolite                                                                             .34           1.71                                               Laumontite   .34           1.77                                               ZSM-4        .38           1.65                                               Heulandite   .39           1.69                                               P            .41           1.57                                               Offretite    .40           1.55                                               Levynite     .40           1.54                                               Erionite     .35           1.51                                               Gmelinite    .44           1.46                                               Chabazite    .47           1.45                                               A            .5            1.3                                                Y            .48           1.27                                               ______________________________________                                    

Members of the above group of zeolites for use in the catalystcomposition of the present invention possess definite distinguishingcrystalline structures as evidenced by the above U.S. Patentsincorporated herein by reference.

Zeolites ZSM-5, ZSM-11, ZSM-12, ZSM-35 and ZSM-38 for use in the processof this invention are prepared as indicated in the patents incorporatedherein by reference above.

The zeolite described in U.S. application Ser. No. 878,558, filed Feb.17, 1978 can be prepared utilizing materials which supply theappropriate components of the zeolite. Such components include sodiumaluminate, alumina, sodium silicate, silica hydrosol, silica gel,silicic acid, sodium hydroxide and a tetrapropylammonium compound, e.g.,tetrapropylammonium hydroxide. It will be understood that each componentutilized in the reaction mixture for preparing the zeolite can besupplied by one or more initial reactants and they can be mixed togetherin any order. For example, sodium can be supplied by an aqueous solutionof sodium hydroxide, or by an aqueous solution of sodium silicate;tetrapropylammonium cation can be supplied by the bromide salt. Thereaction mixture can be prepared either batchwise or continuously.Crystal size and crystallization time of the composition will vary withthe nature of the reaction mixture employed. It will be furtherunderstood that in the very high silica-to-alumina ratios, which can forthis zeolite range from greater than 35 to about 3000 or more, andpreferably from about 70 to about 500, it may not be necessary to add asource of alumina to the reaction mixture since residual amounts inother reactants may suffice.

The zeolite described in U.S. application Ser. No. 878,558 may beprepared from a reaction mixture having a composition, in terms of moleratios of oxides or in moles of oxides, falling within the followingranges:

    ______________________________________                                                                      Most                                                        Broad   Preferred Preferred                                       ______________________________________                                        OH.sup.- /SiO.sub.2                                                                         0.07-1.0  0.1-0.8   0.2-0.75                                    R.sub.4 N.sup.+ /(R.sub.4 N.sup.+  + Na.sup.+)                                              0.2-0.95  0.3-0.9   0.4-0.9                                     H.sub.2 O/OH.sup.-                                                                          10-300    10-300    10-300                                      SiO.sub.2 /Al.sub.2 O.sub.3                                                                 50-3000   70-1000   70-500                                      Other Metal Oxide                                                             (% of total Oxides)                                                                         1×10.sup.-6 -1.0                                                                  1×10.sup.-5 -0.1                                                                  1×10.sup.-5 -0.01                     ______________________________________                                    

wherein R is as above defined.

Typical reaction conditions for preparation of the zeolite described inU.S. application Ser. No. 878,558 consist of heating the foregoingreaction mixture to a temperature of from about 95° C. to 175° C. for aperiod of time of from about six hours to 120 days. A more preferredtemperature range is from about 100° C. to 175° C. with the amount oftime at a temperature in such range being from about 12 hours to 8 days.The digestion of the gel particles is carried out until crystals form.The solid product is separated from the reaction medium, as by coolingthe whole to room temperature, filtering and water washing. Theforegoing product is dried, e.g. at 110° C., for from about 8 to 24hours. Of course, milder conditions may be employed if desired, e.g.room temperature under vacuum.

For the improved isomerization process of this invention, the suitablezeolite catalyst is employed in combination with a support or bindermaterial which acts as diluent such as, for example, a porous inorganicoxide support or a clay binder. Non-limiting examples of such bindermaterials include alumina, zirconia, silica, magnesia, thoria, titania,boria and combinations thereof, generally in the form of dried inorganicoxide gels and gelatinous precipitates. Suitable clay materials include,by way of example, bentonite and kieselguhr. The relative proportion ofsuitable crystalline aluminosilicate zeolite of the total composition ofcatalyst and binder or support may vary with the zeolite content rangingfrom between about 10 to about 90 percent by weight and more usually inthe range of about 20 to about 80 percent by weight of the composition.

The operating conditions employed in the improved process of the presentinvention are important. Such conditions as temperature, pressure, spacevelocity, molar ratio of the reactants, hydrogen to hydrocarbon moleratio, and the presence of any feedstock diluents, such as tolueneand/or C₉ ⁺ recycle material, will have important effects on theprocess.

The process of this invention is conducted such that isomerization ofthe monocyclic methyl-substituted aromatic hydrocarbon of from 8 to 10carbon atoms contained in a feedstock which also contains ethylbenzeneis carried out in the vapor phase by contact in a reaction zone, suchas, for example, a fixed bed, with the above-described catalyst underisomerization effective conditions, said catalyst being characterized,as synthesized, as containing the above defined zeolite with may or maynot have been hydrogen or hydrogen precursor exchanged and Group VIIImetal exchanged. This process may be conducted in either fixed or fluidbed operation with attendant benefits of either operation readilyobtainable.

The present improved isomerization process is carried out at a pressureof from about 0 psig to about 500 psig. The weight hourly space velocity(WHSV) based on weight of total catalyst is maintained at from about 0.1hr⁻¹ to about 200 hr⁻¹, and the hydrogen/hydrocarbon mole ratio at from0 to about 10, said hydrogen/hydrocarbon mole ratio being 0 when thezeolite component of the catalyst contains hydrogen or hydrogenprecursor cations and from about 0.1 to about 10 when said zeolitecontains Group VIII metal cations. The temperature during the first stepof the present process must be maintained at a level which provides fromabout 50 to about 75 mole percent conversion of ethylbenzene in thefeedstock. Said temperature during this first step is normally fromabout 335° C. to about 370° C. The temperature during the second step ofthe process is then maintained at a level which provides from about 15to less than about 50 mole percent ethylbenzene conversion, preferablyfrom about 15 to about 30 mole percent ethylbenzene conversion. Saidtemperature during the second step is normally within the range of fromabout 260° C. to about 370° C.

It is interesting to note that at the start of a vapor phaseisomerization process under the above conditions and when xylene is thehydrocarbon feedstock, a xylene loss of 2.5 mole percent per pass ischaracteristic at an ethylbenzene conversion of 25 mole percent.Preconditioning the catalyst by way of the first step of the presentimproved process allows subsequent xylene losses to be substantiallyreduced with only an insignificantly small, if any, activity lossnoticeable.

The feedstock to be employed in the present improved process contains,along with ethylbenzene, single ring aromatic hydrocarbons containing aminimum of two and a maximum of four methyl group substituents on thering. These feed materials may be illustrated by the followingstructural formula: ##STR1## wherein R is methyl and n is an integer of2 to 4. The amount of ethylbenzene to be found in the feedstock willnormally vary from about 2 to about 30 weight percent.

Specific compounds falling within the above structural formula includepara-xylene, meta-xylene, ortho-xylene, mesitylene (1, 3,5-trimethylbenzene), durene (1, 2, 4, 5-tetramethylbenzene),hemimellitene (1, 2, 3-trimethylbenzene), pseudocumene (1, 2,4-trimethylbenzene), prehnitene (1, 2, 3, 4-tetramethylbenzene) andisodurene (1, 2, 3, 5-tetramethylbenzene).

Of the above listing of specific feed materials which may be used, thexylene isomers and pseudocumene are especially preferred.

The specific examples, hereinafter discussed, will serve to illustratethe process of the present invention without unduly limiting same.

EXAMPLE 1

A composite zeolite catalyst for use in the present process was preparedas follows:

A sodium silicate solution was prepared by mixing 16 parts water and27.7 parts sodium silicate (28.7 wt. % SiO₂, 8.9 wt. % Na₂ O, 62.4% H₂O). The solution was cooled to approximately 15° C.

An acid solution was prepared by adding 1 part aluminum sulfate (17.2wt. % Al₂ O₃) to 16.4 parts water followed by 2.4 parts sulfuric acid(93 wt. % H₂ SO₄) and 1.2 parts NaCl.

These solutions were mixed in an agitated vessel while 3.9 parts of NaClwere added. The gel molar ratios expressed as oxides are the following:

    SiO.sub.2 /Al.sub.2 O.sub.3 =78.4

    na.sub.2 O/Al.sub.2 O.sub.3 =49.9

an organic solution was prepared by adding 1.6 parts n-propyl bromideand 3.1 parts methyl ethyl ketone to 1.9 parts tri-n-propylamine.

After the gel was heated to about 95°C., agitation was reduced and theorganic solution was added above the gel. This mixture was held at about95°-110° C. for 14 hours, then agitation increased and the temperaturewas increased to about 150°-160° C. and held there until crystallizationwas complete. Unreacted organics were removed by flashing and theremaining contents cooled.

The zeolite slurry product was diluted with 4-5 parts water per partslurry, allowed to settle and supernatant liquid was drawn off. Thesettled solids were reslurried to the original volume of the precedingstep with water. After settling, the aqueous phase was decanted. Thisprocedure was repeated until the sodium level of the zeolite was lessthan 1.0 wt. %. The washed zeolite was then filtered, dried andidentified as ZSM-5 having a silica/alumina mole ratio of at least 12,i.e. about 70, and a constraint index of between 1 and 12, i.e. about8.3.

The dried zeolite was then mixed with alumina and water. It was thenextruded into 1/16" pellets and dried. The extruded material contained65 parts ZSM-5 per 35 parts alumina.

The dried extrudate was calcined for three hours at 538° C. in flowingnitrogen. After cooling, the extrudate was contacted with an ammoniumnitrate exchange solution (about 0.08 lb. NH₄ NO₃ /lb extrudate) for onehour at ambient temperature. This exchange was then repeated until thesodium level was less than 0.05 wt. %. The extrudate was then contactedwith a nickel nitrate exchange solution (about 0.1 lb Ni(NO₃)₂.6H₂ O/lbextrudate) for two hours at about 80°-90° C. After this exchange, theextrudate was washed, dried and calcined in a flowing 10% air- 90%nitrogen gas mixture at 538° C. for six hours.

EXAMPLE 2

A 25 cc quantity of the catalyst material of Example 1 was placed in anisothermal, one gallon/day reactor. Pure hydrogen gas was continuouslypassed through the reactor without recycle in order to maintain aconstant molar ratio between hydrogen and hydrocarbon feedstock at thereactor inlet. A feedstock comprised of the components listed in Table1, hereinafter presented, was then passed through the reactor at atemperature of 330° C., a pressure of 200 psig, a weight hourly spacevelocity of 7 hr⁻¹ and a hydrogen/hydrocarbon mole ratio of 4. At anethylbenzene conversion of about 25 mole percent, the xylene loss wasmeasured to be 2.46 mole percent.

                  TABLE 1                                                         ______________________________________                                        Feedstock Composition for Example 2                                           Component        Amount, wt. %                                                ______________________________________                                        Toluene          0.1                                                          Ethylbenzene     14.8                                                         p-Xylene         10.3                                                         m-Xylene         67.1                                                         o-Xylene         7.6                                                          C.sub.9.sup. + Aromatics                                                                       0.1                                                          ______________________________________                                    

EXAMPLE 3

Example 2 was repeated except for the temperature being 360° C. for 24hours at the beginning of the experiment. During this time period theethylbenzene conversion was 50 mole percent. The temperature was thenreduced to 330° C. with an accompanying reduction in ethylbenzeneconversion to about 25 mole percent. At this reduced temperature, thesame as for Example 2, xylene losses were measured to be only 2.04 molepercent, a 17 percent improvement over the results obtained in Example2.

EXAMPLE 4

At the conclusion of Example 3, the temperature was raised to 360° C.for an additional 48 hours. During this time period the ethylbenzeneconversion was 50 mole percent. The temperature was then reduced to 330°C., the same as for Example 2, with an accompanying reduction inethylbenzene conversion to about 25 mole percent. At this reducedtemperature, xylene losses were measured to be only 1.70 mole percent, a30.9 percent improvement over the results obtained in Example 2.

What is claimed is:
 1. In a process for effecting vapor phase catalyticisomerization of monocyclic methyl-substituted aromatic hydrocarboncompounds of from 8 to 10 carbon atoms contained in a feedstock alsocontaining ethylbenzene to cause rearrangement of alkyl groups in saidcompounds which comprises contacting said feedstock in the vapor phasewith a catalyst containing crystalline aluminosilicate zeolitecharacterized by a constraint index within the approximate range of 1 to12, said zeolite containing cations which are selected from the groupconsisting of hydrogen, hydrogen precursor and metal of Group VIII ofthe Periodic Table of Elements, at a temperature of from about 260° C.to about 370° C., a pressure of from about 0 psig to about 500 psig, ahydrogen/hydrocarbon mole ratio of from 0 to about 10 and a weighthourly space velocity of from about 0.1 hr⁻¹ to about 200 hr⁻¹, saidhydrogen/hydrocarbon mole ratio being 0 when said cations are hydrogenor hydrogen precursor and from about 0.1 to about 10 when said cationsinclude a metal of Group VIII of the Periodic Table of Elements, theimprovement wherein said process comprises a first step wherein thetemperature is maintained at a level which provides from about 50 molepercent to about 75 mole percent conversion of said ethylbenzene for aperiod of time of from about 24 hours to about 96 hours, therebyselectively coking the catalyst to a controlled extent, and a secondstep wherein the temperature is maintained at a level which providesfrom about 15 mole percent to less than about 50 mole percent conversionof said ethylbenzene.
 2. The process of claim 1 wherein said zeolite isZSM-5, ZSM-11, ZSM-12, ZSM-35, ZSM-38 or one identified, in terms ofmole ratios of oxides in the anhydrous state, as follows:

    (R.sub.2 O, M'.sub.2/n O).sub.W :(Al.sub.2 O.sub.3).sub.X :(SiO.sub.2).sub.Y :(M".sub.2/n O).sub.Z

wherein W/X is from greater than 0.5 to less than 3, Y/X is greater than20 and Z/X is from greater than zero to less than about 100, R is anitrogen-containing cation and n is the valence of M' and M".
 3. Theprocess of claim 2 wherein said zeolite is ZSM-5.
 4. The process ofclaim 1 wherein said zeolite is combined in an amount of from about 10to about 90 weight percent in a binder therefor.
 5. The process of claim4 wherein said binder is alumina.
 6. The process of claim 2 wherein saidzeolite is combined in an amount of from about 10 to about 90 weightpercent in a binder therefor.
 7. The process of claim 6 wherein saidbinder is alumina.
 8. The process of claim 3 wherein said zeolite iscombined in an amount of from about 10 to about 90 weight percent in abinder therefor.
 9. The process of claim 8 wherein said binder isalumina.
 10. The process of claim 1 wherein said Group VIII metalcations are selected from the group consisting of nickel, iron, cobaltand mixtures thereof.
 11. The process of claim 10 wherein said GroupVIII metal cations are nickel.
 12. The process of claim 2 wherein saidGroup VIII metal cations are selected from the group consisting ofnickel, iron, cobalt and mixtures thereof.
 13. The process of claim 12wherein said Group VIII metal cations are nickel.
 14. The process ofclaim 3 wherein said Group VIII metal cations are selected from thegroup consisting of nickel, iron, cobalt and mixtures thereof.
 15. Theprocess of claim 14 wherein said Group VIII metal cations are nickel.16. The process of claim 1 wherein said feedstock contains materialsillustrated by the formula: ##STR2## wherein R is methyl and n is aninteger of from 2 to
 4. 17. The process of claim 2 wherein saidfeedstock contains materials illustrated by the formula: ##STR3##wherein R is methyl and n is an integer of from 2 to
 4. 18. The processof claim 3 wherein said feedstock contains materials illustrated by theformula: ##STR4## wherein R is methyl and n is an integer of from 2 to4.
 19. The process of claim 10 wherein said feedstock contains materialsillustrated by the formula: ##STR5## wherein R is methyl and n is aninteger of from 2 to
 4. 20. The process of claim 12 wherein saidfeedstock contains materials illustrated by the formula: ##STR6##wherein R is methyl and n is an integer of from 2 to
 4. 21. The processof claim 14 wherein said feedstock contains materials illustrated by theformula: ##STR7## wherein R is methyl and n is an integer of from 2 to4.
 22. The process of claim 16 wherein said feedstock contains xylenes.23. The process of claim 17 wherein said feedstock contains xylenes. 24.The process of claim 18 wherein said feedstock contains xylenes.
 25. Theprocess of claim 1 wherein said second step temperature is maintained ata level which provides from about 15 mole percent to about 30 molepercent conversion of said ethylbenzene.
 26. The process of claim 2wherein said second step temperature is maintained at a level whichprovides from about 15 mole percent to about 30 mole percent conversionof said ethylbenzene.
 27. The process of claim 3 wherein said secondstep temperature is maintained at a level which provides from about 15mole percent to about 30 mole percent conversion of said ethylbenzene.